38 November/December 2021
Physics
8theorem, that new black hole should be equal in
area to the total area of the original two black holes.
That’s exactly what the researchers found per the
wave data of the observed inspiraling system.
The confirmation brings us closer to under-
standing the intricacies of black hole functions,
says Riccardo Penco, Ph.D., assistant Professor of
High Energy Physics Theory at Carnegie Mellon
University. “The detection of gravitational waves
allows us to test black holes directly (as opposed to
just inferring their existence indirectly by studying
the motion of other objects around them). For the
longest time, we have studied black holes as math-
ematical objects endowed with a remarkable set of
properties (Hawking’s area law being one of them).
We are now in a position to test these properties
experimentally.”
Per Hawking’s calculations, if a black hole
changes in area, its event horizon would stretch or
constrict in correspondence with its new size. The
event horizon, the area of effect for a black hole’s
gravitational field, is the point of no return for any-
thing—including light—orbiting the black hole. An
object would need to travel faster than the speed of
light in order to escape.
Penco says Hawking’s Theorem holds true for
the black holes observed by the Laser Interferom-
eter Gravitational-Wave Observatory (LIGO)—a
pair of U.S. facilities built to study gravitational
waves—whose work these researchers used for their
findings. But Hawking’s Theorem could apply to
other black holes as well, those that at first seem
to defy the Theorem until they prove applicable to
another concept hypothesized by the physicist.
“Hawking showed that when quantum mechan-
ical effects [like the behavior of subatomic matter
and light] are taken into account, a black hole can
spontaneously reduce its surface area over time by
emitting radiation,” says Penco. Black hole evapo-
ration, also called Hawking radiation (see sidebar),
is theorized electromagnetic radiation sponta-
neously expelled by black holes. So Hawking’s
Area Theorem has an exception, Hawking radia-
tion, touted by Hawking himself.
LIGO detected the first gravitational wave sig-
nal in this study—essentially a ripple in space-time
resulting from cosmic interactions—in 2015. The
signal, they discovered, was the product of two
merging black holes (which typically cause the
stronger gravitational waves), along with a vast
amount of energy that rippled across the space-
time continuum.
The researchers posited that if Hawking’s Area
Theorem held up, the event horizon area of the
new black hole created from the merger would not
be smaller than the total event horizon area of its
parent black holes. To test their hypothesis, they
split up the gravitational wave data from the sys-
tem into t wo sections—before the merger and after
the merger—then analyzed both sections to see
how their event horizon areas compared. The sci-
entists then reanalyzed the wave data and found
that the two areas are statistically the same—the
total event horizon area did not decrease postcol-
lision—within a 95 percent confidence margin.
It would seem from the analysis, then, that the
laws of “regular” physics, such as the law of con-
ser vation of mass, can still be applied to the study
of black holes, where physics can become more
theoretical in situations unfamiliar to the Earth-
bound human experience. Working in reverse,
observing real-life data to interrogate the laws
that might govern black hole behavior, could be
a practical way to advance the entire field of cos-
mology. With this black hole mergence, other
scientists now have a concrete example to refer-
ence as they continue to study space.
Still, Penco says this “doesn’t fundamentally
alter our understanding of black holes [ just] because
it confirms our theoretical expectations.” Con-
firming a theory helps us better understand the
mechanics of mysterious cosmic phenomena such
as black holes, but it also creates a more import-
ant tie between the tactile scientific experience
on Earth and the theoretical science of space. “It’s
always important to be able to confirm experimen-
tally,” Penco says. “It’s another step forward toward
a deeper understanding of black holes.”The Lowdown on Hawking Radiation
Proposed in 1974, Hawking radiation
might cause a black hole to shrink.
Subatomic particle pairs near the event
horizon might split, with one particle—
a photon or neutrino, perhaps—
escaping and another, of negative
energy, disappearing into the blackhole. This influx of negative energy
could eventually reduce the black hole’s
mass until it disappears. This means
a black hole’s size is inversely
proportional to its temperature—
the smaller the black hole, the
warmer it is.—Daisy Hernandez